1. Field of the Invention
This invention relates to devices for discharging liquid droplets and more particularly to a droplet-jet system for discharging liquid such as ink as liquid droplets by utilizing thermal energy.
2. Description of the Prior Art
In the prior art droplet-jet system, the amount of liquid or ink in the ink reservoir or the presence of ink in the ink paths connecting the reservoir with the ink-discharging nozzles is detected for the integrity of ink supply to continue the proper discharge of ink droplets. Consequently, it is impossible or uncertain to find out whether the nozzle is filled or unfilled with ink and how much ink remains in the system, when entrainment of bubble or interruption of ink flow is caused by shock or vibration.
In other words, the detection of presence of ink in each nozzle is inferred from the detection in the ink paths or the ink reservoir. Therefore, the detection by the prior art is uncertain.
Some means of detecting ink in each nozzle, devised for solving problem are to observe the nozzle with the eye, to observe the recorded ink dot with the eye or an optical sensor, or to observe flying ink droplets with an optical sensor.
However, the observation of ink dots with the eye or the optical sensor separately from one another is an extremely troublesome and mistakable thing since the distribution of nozzles is highly densified and the diameter of each ink dot is very small. The observation with an optical sensor needs an apparatus of generous size and of increased cost for detecting a minute liquid droplet. In addition, this method is subject to effects of extrinsic factors.
In droplet-jet systems utilizing thermal energy to discharge liquid droplets, when orders to discharge are given to a nozzle unfilled with ink on account of the entrainment of bubble or interruption of ink supply, the electrothermal energy converter and the vicinity thereof undergo an undesirable influence of heating, often deteriorating the performance of the nozzle and eventually leading the whole liquid droplet-discharging head to distruction. Accordingly, it is very important to detect exactly whether each nozzle is filled with ink.